A wavelength division multiplexing (WDM) optical transmission system may transmit a large amount of information by multiplexing signal light modulated by various modulation schemes. The modulation schemes for modulating signal light in the WDM optical transmission system include optical phase modulation schemes and the like. The optical phase modulation schemes are methods of transmitting and receiving information by changing phases of light and are used for transceivers with bit rates of approximately 40 Gbps, for example.
The optical phase modulation schemes include a differential phase shift keying (DPSK) modulation scheme and a differential quadrature phase shift keying (DQPSK) modulation scheme. In recent years, a modulation scheme that is referred to as a dual polarization quadrature phase shift keying (DP-QPSK) modulation scheme has been developed. The DP-QPSK modulation scheme enables information to be transmitted and received at approximately 100 Gbps by multiplexing polarized waves, for example.
It is known that, in an optical transmission system that employs an optical phase modulation scheme, when an optical signal modulated by the optical phase modulation scheme is output from an optical transmitter to an optical transmission path, the optical signal transferred by the optical transmission path is deteriorated due to various causes. For example, the optical signal is deteriorated by Amplified Spontaneous Emission (ASE) light of an optical amplifier installed in the optical transmission path. In addition, the optical signal is deteriorated by chromatic dispersion, polarization mode dispersion, a polarization-dependent loss, state-of-polarization rotation, and the like. Furthermore, it is known that the optical signal is deteriorated by a nonlinear optical effect of the optical transmission path. The nonlinear optical effect of the optical transmission path is self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing, or the like, for example.
There is a conventional technique for using a known transfer function to extract optical intensity noise generated due to the ASE light, the chromatic dispersion, the polarization mode dispersion, the polarization-dependent loss, the state-of-polarization rotation, and the like that are the causes of the deterioration of the optical signal.
An example of related art is Japanese Laid-open Patent Publication No. 2011-182198.
In the conventional technique, however, the extraction of optical phase noise generated due to the nonlinear optical effect of the optical transmission path is not considered. Thus, there has been a demand to accurately extract the optical phase noise generated due to the nonlinear optical effect of the optical transmission path.
Techniques disclosed herein were devised in consideration of the aforementioned facts, and it is an object of the techniques disclosed herein to provide an optical phase noise extracting device and an optical phase noise extraction method that enable optical phase noise generated due to a nonlinear optical effect of an optical transmission path to be accurately extracted.